1. Technical Field
This disclosure generally relates to communication circuitry.
More specifically, this disclosure relates to circuitry for matching the up and down impedances of a voltage-mode transmitter.
2. Related Art
Modern communication devices use voltage-mode transmitters partly because they use less power than their current-mode counterparts. Most high-speed communication standards impose stringent requirements on the characteristics of voltage-mode transmitters. Specifically, some standards require that a transmitter must not only output a differential voltage at a desired magnitude, but that the transmitter's output impedance must match the impedance of the signal traces of the channel on the circuit board in order to minimize reflections which can impair signal integrity.
A voltage-mode transmitter's output can be viewed as having two impedances: an up impedance looking up to the power supply voltage (e.g., VTT), and a down impedance looking down to ground. In addition to ensuring that the overall impedance of the transmitter matches a desired value (e.g., the impedance of the signal traces in the circuit board), the up and down impedances must also match each other. A mismatch between the up and down impedances of a transmitter can cause a mismatch in the rise and fall times of the differential voltage signal, and can cause excessive common mode noise. This, in turn, can degrade the transmitter's performance and can violate the specifications of the high-speed communication standard.
The relentless miniaturization of integrated circuits has increased the variability in the electrical characteristics of circuit elements. Moreover, the behavior of circuit elements changes with changing operating conditions. Hence, even if the transmitter's up and down impedances match at nominal operating conditions (e.g., at the nominal voltage and temperature), the impedances may not match once the operating conditions change.
Hence, what is needed are systems and techniques for ensuring that the overall impedance of a transistor matches a desired value, and that the up and down impedances of the transmitter match each other.